1. Field of the Invention
The present invention generally relates to a phase shift mask, and more specifically to a tri-tone phase shift mask comprising a quartz substrate region, a phase shift region having a transparency of 100% and a half-tone phase shift region.
2. Description of the Prior Art
Various types of phase shift masks are used to improve the limit in resolution in a lithography process of fabricating process of semiconductor devices.
In the lithography process, an image is obtained by interference of a light of zero order and a light of first order. A diffracted light is a plane wave which does not contain any information on a mask. A pattern on the mask is transferred to a wafer by interference of two diffracted lights.
Here, an image contrast is a ratio of intensities of the light of zero order to that of the light of first order. When the intensities of two diffracted lights are analogous, complete interference occurs and the image contrast is at its maximum value of 1. The intensity of the light of zero order is the average amplitude of the entire lights.
While all of the light of zero order can be transmitted through a lens, the light of first order has a small effective transmission ratio into the lens. As a result, the image contrast is less than 1, and decreases as the size of the patterns is decreased.
The amount of captured light of first order by the lens is determined by a simple geometry, and related to a repetition cycle of the pattern and an illumination method of a system. A rim-type phase shift mask or a half-tone phase shift mask which employs a method wherein the amount of the light of first order is increased and the amount of light of zero order is decreased has been used.
FIG. 1 is a cross-sectional diagram illustrating a rim-type phase shift mask.
Referring to FIG. 1, the rim-type phase shift mask comprises a quartz substrate 11 which includes a trench-type guard ring pattern 13 having a 180° phase shift, and a chrome layer 15 surrounded by the guard ring pattern 13. The chrome layer 15 is disposed on a predetermined region of the quartz substrate 11 where a light-shielding pattern is to be formed.
FIG. 2 is a cross-sectional diagram illustrating a half-tone phase shift mask.
Referring to FIG. 2, the half-tone phase shift mask comprising the quartz substrate 11 and a half-tone phase shift pattern 17 disposed on the quartz substrate 11 restricts the intensity of the light of zero order to improve the image contrast.
FIG. 3a is a graph illustrating the distribution of the intensity of a light of zero order and a light of first order of a BIM (Binary Intensity Mask). FIG. 3b is a graph illustrating the distribution of the intensity of a light of zero order and a light of first order of a half-tone phase shift mask having 6% transparency. FIG. 3c is a graph illustrating the intensity distribution of a light of zero order and a light of first order of a half-tone phase shift mask having 18% transparency.
As shown in FIGS. 3a through 3c, as the transparency of the half-tone phase shift mask is increased, the intensity of the light of zero order is decreased, and the intensity of the light of first order is increased.
The intensity of the light of zero order can be decreased as the transparency of the half-tone phase shift mask is increased. However, since the amount of a transmitted light is large, diffractions and interferences occur in a high density pattern region and desired patterns are formed while it is difficult to form a pattern in a low density pattern region due to direct transmission of the light.